This investigation is concerned with host-parasite interactions occurring in Listeria monocytogenes infections. Iron metabolism in the host and in the bacterium is being studied for its role in the disease. Preparative fractionation and electrophoretic methods for isolation of in vivo grown Listeria cells are being employed to obtain comparative data on in vitro and in vivo grown bacteria. The nature of the toxic and lethal action of Listeria hemolysin is under investigation. Participation of hemolysin and other products of Listeria is being evaluated in experimental listeric infections. Biochemical alterations occurring in blood and other tissues during progressive infection are under investigation.